1. Trying to keep ife simple: if you are on a Cat 2/3 approved rwy for autolands the a/c will follow the ILS G/S to touchdown; so why not do the same on a visual day and manual landing if there is a difference between it and PAPI.
2. In the old days of VASIS 2 & 3 bar we flew 2 red 1 white on 3 bar systems in a medium a/c. I always wondered why not the same on PAPI; 3 red 1 white.
3. I was told by a CAA inspector that an airfield operator had discretion how they set up the PAPI. The angle was published but not the touchdown point. This could be set by the operator for the most critical type, or most common type, of a/c they expected so as to give a safe THR crossing HT. This touchdown point is not published, but by nature if it is safe for a BIG a/c it is safe for a smaller one. However, the ILS G/S remained the primary GP guidance aid for all a/c, and it was this which was flight checked to the more critical tolerances.
Having said that, a final visual segment is pilot controlled. Mostly I have found that, if there is a difference, it will bring me high on PAPIs not low. Diving at the rwy from 200' when in trim on G/S does not seem the best idea. Does anyone have experience of the ILS G/S bringing you low on PAPIs? And if so is it worse than 3 reds? I know there are wavy G/S's, but these do not count. This is where the is quite a difference passing 500', but they recover to be the same at DA.